This invention relates to a process for electrolytic production of lithium which comprises electroreducing a lithium compound in an electrolytic cell comprising a fused salt electrolyte, a lithium compound dispersed in said electrolyte, and a liquid metal cathode. More particularly, this invention relates to a process for electrolytic production of lithium which comprises electroreducing lithium oxide in an electrolytic cell comprising a fused salt electrolyte, lithium oxide dispersed in said electrolyte, and a liquid metal cathode.
The production of lithium is usually carried out by electrolysis of lithium compounds and deposition of the lithium at a solid cathode and evolution of a halide gas at the anode. Commercial electrolytic production of lithium is accomplished by electroreduction of a lithium halide in an electrolyte of lithium halide-alkali halide or other lithium compound (see, for example, Cooper et al., U.S. Pat. No. 4,156,635; Thieler, U.S. Pat. No. 3,344,049; Cook et al., U.S. Pat. No. 3,489,659; Hall, U.S. Pat. No. 3,826,721; and Rare Metals Handbook, 2d Edition, C. A. Hamper, editor, page 244, Krieger Pub. Co., Huntington, N.Y., 1971)). Typically electrolysis results in deposition of lithium at the solid cathode and evolution of chlorine gas at the anode. Although the commercial process is a well established method, it has several disadvantages such as: (1) the necessity of using high purity lithium halides, usually chlorides, as a feed material; (2) the electroreduced lithium rises to the top of the electrolyte, along with the evolved halogen gas, and the halogen gas has a tendency to recombine with the lithium unless great care is taken to prevent contact between the lithium and the halogen gas; (3) the evolved halogen gas must be recovered or absorbed rather than vented to the atmosphere; (4) along with lithium, impurities are codeposited at the solid cathode and three impurities are difficult to separate from the lithium; and (5) the lithium and alkali metal halides in the electrolyte are quite hygroscopic (particularly the chlorides), and the absorbed water in the halides reacts with the product lithium to form unwanted lithium oxide.
Gibson et at., U.S. Pat. No. 3,284, 325, and Gibson, Great Britain Patent No. 494,702, disclose the electrolytic production of alkaline earth metals using a liquid metal cathode to absorb the electroreduced alkaline earth metal. The liquid cathode enables separation of impurities from the desired alkaline earth metal, but the patentees disclose only an electrolyte containing potassium chloride and other alkali metal chlorides which are highly hygroscopic. Thus, the absorbed water reacts with the electroreduced alkaline earth metal to form unwanted oxides of the metal. Furthermore, these patents do not disclose either the use of lithium compounds or alkali metal compounds as feed materials or the application of the process to produce lithium or other alkali metals as the product.
Szechtman in U.S. Pat. No. 3,119,664 discloses an electrolytic process for producing alkali metal oxides. As part of this process the patentee discusses the electroreduction of potassium from fused potassium chloride and the use of a molten lead cathode to absorb the electroreduced potassium. Subsequently the alkali metal is separated and oxidized. However, the patentee does not disclose the production of lithium and uses only a chloride electrolyte which is highly hygroscopic. The patent also discloses electroreduction of alkali metal halide feed materials which are difficult and expensive to prepare in the case of lithium. Further, the electroreduction of halide salts results in evolution at the anode of unwanted halogen gases. Also, the salt undergoing electroreduction is a main component of the base electrolyte which requires the monitoring and balancing of electrolyte composition.
Since sodium and potassium are commonly found as chlorides, the prior art has been directed toward electroreduction of these chlorides to the alkali metal. However, lithium is not usually found as a choride, but rather as carbonate, which is easily converted to lithium oxide. Accordingly, there is a need for an electrolytic process to convert lithium carbonate, lithium oxide, or other common lithium compounds to elemental lithium.
The general object of this invention is to provide an improved electrolytic process for the production of lithium from lithium compounds. A more specific object of this invention is to provide a new electrolytic process for the production of lithium from lithium oxide. Other objects of the invention will be apparent to persons skilled in the art from the following description and appended claims.
For purposes of this invention, the phrase "dispersed in a fused salt electrolyte" comprises the physical suspension of the feed in the electrolyte as well as chemical dissolution in the electrolyte. The dispersion should be such that the material is capable of undergoing electroreduction and deposition of lithium at the cathode.